Long Term Evolution (LTE) specifications from the 3rd Generation Partnership Project (3GPP) support component carrier bandwidth up to 20 MHz. However, in order to meet the International Mobile Telecommunications Advanced (IMT-Advanced) requirements for (very) high data rates, the concept of carrier aggregation has been introduced to support bandwidths larger than 20 MHz. The carrier aggregation concept is illustrated in FIG. 1, where five component carriers, or cells, are illustrated, with 20 MHz bandwidth each. In the example of FIG. 1, the total bandwidth available to a mobile terminal is the sum of the bandwidths of the cells, i.e. 100 MHz.
Note that in the context of carrier aggregation, a component carrier also refers to a cell. Hence five components carriers as illustrated in FIG. 1 correspond to five cells.
A terminal or a user equipment (UE) may be configured with a subset of the cells offered by the network and the number of aggregated cells configured for one terminal or UE may change dynamically through time based on for example terminal traffic demand, type of service used by the terminal, system load etc. A cell which a terminal is configured to use is referred to as a serving cell for that terminal. A terminal has one primary serving cell (called PCell) and zero or more secondary serving cells (SCells), the term serving cell includes both the PCell and SCells. Which cell is a terminal's PCell is terminal-specific. The PCell is considered more important and for example some control signaling is handled via the PCell. Hence in case of five components carriers as shown in FIG. 1, the terminal may have one PCell and zero, one, two, three or four SCells. Some control signalling is handled via the PCell, the PCell is an important carrier for the terminal.
It should be noted that although there is a difference in meaning, for the sake of readability the term serving cell will herein and in some cases be replaced by the term cell.
Aside from that the concept of configuration of cells/carriers has been introduced the concept of activation has been introduced for SCells (not for the PCell). Cells may be configured (or deconfigured) using Radio Resource Control (RRC) signaling, which can be slow, and at least SCells can be activated (or deactivated) using a Medium Access Control (MAC) control element, which is much faster. Since the activation process is based on MAC control elements—which are much faster than RRC signaling—an activation/de-activation process can quickly adjust the number of activated cells to match the number that are required to fulfil data rate needed at any given time. Activation therefore provides the possibility to keep multiple cells configured for activation on an as-needed basis.
When a terminal or UE gets configured with a cell it may need to re-tune the radio frontend (RF) to cover the spectrum of the configured cell and to change the carrier frequency. Similarly, when a serving cell is de-configured the terminal may need to re-tune radio frontend so as to not cover the de-configured cell. As a consequence of radio frontend re-tuning the terminal may need to perform an interruption, or glitch, during which the terminal is not able to receive of transmit signals using that radio frontend. An example is shown in FIG. 2 and FIG. 3. In FIG. 2 the terminal is configured with Cell A and Cell B but not Cell C. This is indicated by “covered spectrum”.
In FIG. 3, the terminal is configured with all 3 cells A, B and C. When also Cell C is configured the terminal may need to perform a radio frontend re-tuning and hence perform a glitch or interruption. Similarly with deconfiguration, if the terminal cell configuration is first as in FIG. 3 but at a later stage Cell C is deconfigured the terminal may retune the radio frontend to enter the configuration as in FIG. 2.
When a cell/carrier is activated or deactivated the terminal may also perform a glitch, similar to the case of configuration or deconfiguration.
A glitch may affect all or some of the serving cells of a terminal or UE. Which serving cells are affected by the glitch may depend on how the transceiver architecture looks like or is designed in the terminal and on which radio frontend the different cells are on. When a RF is retuned all serving cells that RF will be affected.
In this disclosure, when it is sometimes said that a terminal is performing a glitch (or similar) it is referred to that the terminal or UE is performing a retuning of one or more of its RF frontends.
Hence, in order for a terminal to be able to use a cell for transmission, the cell first needs to be configured for the terminal. At cell configuration, the terminal may need to perform a glitch, due to RF retuning. When a cell has been configured it needs to be activated before the terminal is able to use it for communication. Also at cell activation the terminal may need to perform a glitch, due to RF retuning.
It is currently discussed in 3GPP the duration of the glitch and it may, in some situations, be as long as 40 ms, which is a non-negligible period of time in this context. During a glitch the terminal or UE is, at least partially, unable to communicate with the network and hence user experience will be degraded as the throughput will be decreased, delay increased and services may be interrupted. So during a glitch, interruption is experienced.